This application claims priority to Korean Patent Application No. 2004-46527 filed on Jun. 22, 2004 in the Korean Intellectual Property Office (KIPO), the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly to a local oscillator for a multi-band wireless communication system.
2. Description of the Related Art
A wireless communication system uses various frequency bands. For example, a 800 MHz frequency band is used for cellular CDMA (Code Division Multiple Access), a 1900 MHz frequency band is used for PCS (Personal Communication Service), and a 1500 MHz frequency band is used for GPS (Global Positioning System).
Generally, a wireless communication terminal uses only one frequency band. For example, a CDMA wireless communication terminal uses about 800 MHz frequency band but not the 1900 MHz PCS frequency band nor the 1500 MHz GPS frequency band.
A multi-band communication system uses two or more frequency bands. For example, a wireless communication receiver is capable of receiving CDMA signals, PCS signals, and GPS signals. The prior art multi-band communication system uses multiple voltage-controlled oscillators for various frequency bands. Therefore, the prior art multi-band communication system has large chip size, high power consumption, and high cost.
A 0-IF (Zero-Intermediate Frequency) communication system does not use an IF (Intermediate Frequency). The 0-IF system is also called a Direct Conversion system. The 0-IF system is opposite to a Super Heterodyne system that uses IF. The 0-IF system directly converts a carrier signal to a baseband signal. In addition, the 0-IF system directly converts a baseband signal to a carrier signal.
The 0-IF system may reduce the number of SAW Filters (Surface Acoustic Wave Filters) and mixers so that the 0-IF system may reduce production cost and device size. The number of wireless communication systems to which the 0-IF system is applied has increased recently.
When a LO (Local Oscillating) signal has almost the same frequency as the frequency of the RF (Radio Frequency) signal, the 0-IF system performs adversely with signal leakage, DC offset, and I/O mismatch. Therefore, the 0-IF system generates an oscillating signal that has a frequency different from the frequency of the RF signal.
For example, a 0-IF multi-band wireless communication system generates oscillating signals that have frequencies different from the RF signal frequency using multiple voltage-controlled oscillators. The 0-IF system divides the frequencies of the oscillating signals to generate frequency divided oscillating signals. The 0-IF system mixes the oscillating signals and the divided oscillating signals with each other to generate the local-oscillating signals having wanted frequencies.
FIG. 1 is a block diagram of a conventional multi-band wireless communication system. Referring to FIG. 1, a PCS signal, a cellular signal, and a GPS signal are received at antennas 111, 112, and 113, respectively. Band pass filters 121, 122, and 123 filter the received PCS signal, cellular signal, and GPS signal, respectively, for filtering away unwanted frequency band signals. The filtered PCS signal, cellular signal, and GPS signal are amplified by LNAs (Low Noise Amplifiers) 131, 132, and 133, respectively. Band pass filters 141 and 142 filter the amplified PCS signal and cellular signal, respectively.
Then, mixers 151, 152, and 153 mix the thus processed PCS signal, cellular signal, and GPS signal with local-oscillating signals LO1, LO2, and LO3, respectively. The local oscillator 100 generates the local-oscillating signals LO1, LO2, and LO3. Therefore, a 0-IF PCS signal, a 0-IF cellular signal, and a 0-IF GPS signal are generated from the mixers 151, 152, and 153. Low pass filters 161, 162, and 163 filter the 0-IF PCS signal, the 0-IF cellular signal, and the 0-IF GPS signal, respectively. A modem 170 modulates or demodulates the filtered 0-IF PCS signal, 0-IF cellular signal, and 0-IF GPS signal.
The local oscillator in FIG. 1 includes two voltage-controlled oscillators 101 and 102, a complex LO generator 103, and a PLL (Phase Locked Loop) 104. The voltage-controlled oscillator 101 generates a first frequency band signal at about 1700 MHz. The voltage-controlled oscillator 102 generates a second frequency band signal at about 3150 MHz.
For the cellular signal, the LO generator 103 divides a frequency of the 1700 MHz frequency band signal by two to generate the local-oscillating signal LO2. The frequency range of the local-oscillating signal LO2 is in a range from about 832 MHz to about 894 MHz. For the GPS signal, the LO generator 103 divides a frequency of the 3150 MHz frequency band signal by two to generate the local-oscillating signal LO3. The frequency range of the local-oscillating signal LO3 is about 1575.42 MHz.
For the PCS signal, the LO generator 103 divides a frequency of the 1700 MHz frequency band signal by eight to generate a divided signal. The LO generator 103 mixes the 1700 MHz frequency band signal with the divided signal to generate the local-oscillating signal LO1. The frequency range of the local-oscillating signal LO1 is in a range from about 1840 MHz to about 1990 MHz.
The local oscillator 100 of the prior art uses two voltage-controlled oscillators 101 and 102 and at least one mixer within the LO generator 103 to generate the local-oscillating signals. Thus, the multi-band wireless communication system of FIG. 1 unfortunately has large chip size, high power consumption, high product price, and increased noise floor and spurious frequencies.